[PATCH bpf-next 17/18] bpf, arm64: Add JIT support for stack arguments

Yonghong Song
2026-04-24T17:17:29Z

From: Puranjay Mohan <puranjay@kernel.org>

Implement stack argument passing for BPF-to-BPF and kfunc calls with
more than 5 parameters on arm64, following the AAPCS64 calling
convention.

BPF R1-R5 already map to x0-x4. With BPF_REG_0 moved to x8 by the
previous commit, x5-x7 are free for arguments 6-8. Arguments 9-12
spill onto the stack at [SP+0], [SP+8], ... and the callee reads
them from [FP+16], [FP+24], ... (above the saved FP/LR pair).

BPF convention uses fixed offsets from BPF_REG_PARAMS (r11): off=-8 is
always arg 6, off=-16 arg 7, etc. The verifier invalidates all outgoing
stack arg slots after each call, so the compiler must re-store before
every call. This means x5-x7 don't need to be saved on stack.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
arch/arm64/net/bpf_jit_comp.c | 87 +++++++++++++++++++++++++++++++++--
1 file changed, 83 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 085e650662e3..7adf2b0f4610 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -86,6 +86,7 @@ struct jit_ctx {
__le32 *image;
__le32 *ro_image;
u32 stack_size;
+ u16 stack_arg_size;
u64 user_vm_start;
u64 arena_vm_start;
bool fp_used;
@@ -533,13 +534,19 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
* | |
* +-----+ <= (BPF_FP - prog->aux->stack_depth)
* |RSVD | padding
- * current A64_SP => +-----+ <= (BPF_FP - ctx->stack_size)
+ * +-----+ <= (BPF_FP - ctx->stack_size)
+ * | |
+ * | ... | outgoing stack args (9+, if any)
+ * | |
+ * current A64_SP => +-----+
* | |
* | ... | Function call stack
* | |
* +-----+
* low
*
+ * Stack args 6-8 are passed in x5-x7, args 9+ at [SP].
+ * Incoming args 9+ are at [FP + 16], [FP + 24], ...
*/

emit_kcfi(is_main_prog ? cfi_bpf_hash : cfi_bpf_subprog_hash, ctx);
@@ -613,6 +620,9 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
if (ctx->stack_size && !ctx->priv_sp_used)
emit(A64_SUB_I(1, A64_SP, A64_SP, ctx->stack_size), ctx);

+ if (ctx->stack_arg_size)
+ emit(A64_SUB_I(1, A64_SP, A64_SP, ctx->stack_arg_size), ctx);
+
if (ctx->arena_vm_start)
emit_a64_mov_i64(arena_vm_base, ctx->arena_vm_start, ctx);

@@ -673,6 +683,9 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx)
/* Update tail_call_cnt if the slot is populated. */
emit(A64_STR64I(tcc, ptr, 0), ctx);

+ if (ctx->stack_arg_size)
+ emit(A64_ADD_I(1, A64_SP, A64_SP, ctx->stack_arg_size), ctx);
+
/* restore SP */
if (ctx->stack_size && !ctx->priv_sp_used)
emit(A64_ADD_I(1, A64_SP, A64_SP, ctx->stack_size), ctx);
@@ -1034,6 +1047,9 @@ static void build_epilogue(struct jit_ctx *ctx, bool was_classic)
const u8 r0 = bpf2a64[BPF_REG_0];
const u8 ptr = bpf2a64[TCCNT_PTR];

+ if (ctx->stack_arg_size)
+ emit(A64_ADD_I(1, A64_SP, A64_SP, ctx->stack_arg_size), ctx);
+
/* We're done with BPF stack */
if (ctx->stack_size && !ctx->priv_sp_used)
emit(A64_ADD_I(1, A64_SP, A64_SP, ctx->stack_size), ctx);
@@ -1191,6 +1207,41 @@ static int add_exception_handler(const struct bpf_insn *insn,
return 0;
}

+static const u8 stack_arg_reg[] = { A64_R(5), A64_R(6), A64_R(7) };
+
+#define NR_STACK_ARG_REGS ARRAY_SIZE(stack_arg_reg)
+
+static void emit_stack_arg_load(u8 dst, s16 bpf_off, struct jit_ctx *ctx)
+{
+ int idx = bpf_off / sizeof(u64) - 1;
+
+ if (idx < NR_STACK_ARG_REGS)
+ emit(A64_MOV(1, dst, stack_arg_reg[idx]), ctx);
+ else
+ emit(A64_LDR64I(dst, A64_FP, (idx - NR_STACK_ARG_REGS) * sizeof(u64) + 16), ctx);
+}
+
+static void emit_stack_arg_store(u8 src_a64, s16 bpf_off, struct jit_ctx *ctx)
+{
+ int idx = -bpf_off / sizeof(u64) - 1;
+
+ if (idx < NR_STACK_ARG_REGS)
+ emit(A64_MOV(1, stack_arg_reg[idx], src_a64), ctx);
+ else
+ emit(A64_STR64I(src_a64, A64_SP, (idx - NR_STACK_ARG_REGS) * sizeof(u64)), ctx);
+}
+
+static void emit_stack_arg_store_imm(s32 imm, s16 bpf_off, const u8 tmp, struct jit_ctx *ctx)
+{
+ int idx = -bpf_off / sizeof(u64) - 1;
+
+ emit_a64_mov_i(1, tmp, imm, ctx);
+ if (idx < NR_STACK_ARG_REGS)
+ emit(A64_MOV(1, stack_arg_reg[idx], tmp), ctx);
+ else
+ emit(A64_STR64I(tmp, A64_SP, (idx - NR_STACK_ARG_REGS) * sizeof(u64)), ctx);
+}
+
/* JITs an eBPF instruction.
* Returns:
* 0 - successfully JITed an 8-byte eBPF instruction.
@@ -1646,6 +1697,11 @@ static int build_insn(const struct bpf_verifier_env *env, const struct bpf_insn
case BPF_LDX | BPF_MEM | BPF_H:
case BPF_LDX | BPF_MEM | BPF_B:
case BPF_LDX | BPF_MEM | BPF_DW:
+ if (insn->src_reg == BPF_REG_PARAMS) {
+ emit_stack_arg_load(dst, off, ctx);
+ break;
+ }
+ fallthrough;
case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
case BPF_LDX | BPF_PROBE_MEM | BPF_W:
case BPF_LDX | BPF_PROBE_MEM | BPF_H:
@@ -1671,7 +1727,7 @@ static int build_insn(const struct bpf_verifier_env *env, const struct bpf_insn
}
if (src == fp) {
src_adj = ctx->priv_sp_used ? priv_sp : A64_SP;
- off_adj = off + ctx->stack_size;
+ off_adj = off + ctx->stack_size + ctx->stack_arg_size;
} else {
src_adj = src;
off_adj = off;
@@ -1752,6 +1808,11 @@ static int build_insn(const struct bpf_verifier_env *env, const struct bpf_insn
case BPF_ST | BPF_MEM | BPF_H:
case BPF_ST | BPF_MEM | BPF_B:
case BPF_ST | BPF_MEM | BPF_DW:
+ if (insn->dst_reg == BPF_REG_PARAMS) {
+ emit_stack_arg_store_imm(imm, off, tmp, ctx);
+ break;
+ }
+ fallthrough;
case BPF_ST | BPF_PROBE_MEM32 | BPF_B:
case BPF_ST | BPF_PROBE_MEM32 | BPF_H:
case BPF_ST | BPF_PROBE_MEM32 | BPF_W:
@@ -1762,7 +1823,7 @@ static int build_insn(const struct bpf_verifier_env *env, const struct bpf_insn
}
if (dst == fp) {
dst_adj = ctx->priv_sp_used ? priv_sp : A64_SP;
- off_adj = off + ctx->stack_size;
+ off_adj = off + ctx->stack_size + ctx->stack_arg_size;
} else {
dst_adj = dst;
off_adj = off;
@@ -1814,6 +1875,11 @@ static int build_insn(const struct bpf_verifier_env *env, const struct bpf_insn
case BPF_STX | BPF_MEM | BPF_H:
case BPF_STX | BPF_MEM | BPF_B:
case BPF_STX | BPF_MEM | BPF_DW:
+ if (insn->dst_reg == BPF_REG_PARAMS) {
+ emit_stack_arg_store(src, off, ctx);
+ break;
+ }
+ fallthrough;
case BPF_STX | BPF_PROBE_MEM32 | BPF_B:
case BPF_STX | BPF_PROBE_MEM32 | BPF_H:
case BPF_STX | BPF_PROBE_MEM32 | BPF_W:
@@ -1824,7 +1890,7 @@ static int build_insn(const struct bpf_verifier_env *env, const struct bpf_insn
}
if (dst == fp) {
dst_adj = ctx->priv_sp_used ? priv_sp : A64_SP;
- off_adj = off + ctx->stack_size;
+ off_adj = off + ctx->stack_size + ctx->stack_arg_size;
} else {
dst_adj = dst;
off_adj = off;
@@ -2065,6 +2131,14 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
ctx.user_vm_start = bpf_arena_get_user_vm_start(prog->aux->arena);
ctx.arena_vm_start = bpf_arena_get_kern_vm_start(prog->aux->arena);

+ if (prog->aux->stack_arg_depth > prog->aux->incoming_stack_arg_depth) {
+ u16 outgoing = prog->aux->stack_arg_depth - prog->aux->incoming_stack_arg_depth;
+ int nr_on_stack = outgoing / sizeof(u64) - NR_STACK_ARG_REGS;
+
+ if (nr_on_stack > 0)
+ ctx.stack_arg_size = round_up(nr_on_stack * sizeof(u64), 16);
+ }
+
if (priv_stack_ptr)
ctx.priv_sp_used = true;

@@ -2229,6 +2303,11 @@ bool bpf_jit_supports_kfunc_call(void)
return true;
}

+bool bpf_jit_supports_stack_args(void)
+{
+ return true;
+}
+
void *bpf_arch_text_copy(void *dst, void *src, size_t len)
{
if (!aarch64_insn_copy(dst, src, len))
--
2.52.0